Memory system and controller

ABSTRACT

In a memory system in an embodiment, in a case of normal operation, a control unit returns a write completion response upon completion of reception of write data from a host, and writes the write data into nonvolatile memory in a multiple values. In a case of unordinary power-off, changeover to operation using a backup battery is conducted and the control unit writes dirty data that is not completed in writing into the nonvolatile memory, into the nonvolatile memory with two values. When next boot, the control unit reads the dirty data from the nonvolatile memory into the volatile memory, and thereafter writes the dirty data into the nonvolatile memory in a multiple values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is continuation of application Ser. No. 16/296,720,filed Mar. 8, 2019, which is a continuation of application Ser. No.15/801,572, filed Nov. 2, 2017, which is a continuation of applicationSer. No. 14/656,352, filed Mar. 12, 2015 and is based upon and claimsthe benefit of priority from U.S. Provisional Application No.62/048,663, filed on Sep. 10, 2014 the entire contents of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a memory system and acontroller.

BACKGROUND

In general, in a memory system such as an SSD (Solid State Drive), writedata requested to write by a host is written into nonvolatile memory. Inthe memory system at this time, the write data is written into thenonvolatile memory via volatile memory such as a buffer.

Even in a case where sudden power shut down has occurred, in such amemory system, it is necessary in some cases to store write data forwhich a response of write completion is given to a host, intononvolatile memory. In the memory system, therefore, data in volatilememory is written into nonvolatile memory by using a backup battery.

In a case where large power is used at the time of writing, however, thebackup battery becomes large in size and high in cost. Therefore, it isdesired to reduce the size and cost of the backup battery by makingwrite data nonvolatile with low power when sudden power shut down hasoccurred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a memory systemaccording to an embodiment;

FIG. 2 is a diagram used to explain an operation processing procedure ina memory system according to an embodiment;

FIG. 3 is a flow chart illustrating a PLP processing procedure in amemory system according to an embodiment when sudden power shut down hasoccurred;

FIG. 4 is a flow chart illustrating a PLP processing procedure conductedby an FE in an embodiment;

FIG. 5 is a flow chart illustrating a PLP processing procedure conductedby an BE in an embodiment;

FIG. 6 is a flow chart illustrating a cache processing procedure ofwrite data;

FIG. 7 is a flow chart illustrating a processing procedure of flushprocessing at the time of normal operation;

FIG. 8 is a flow chart illustrating a PLP processing procedure in a casewhere PLP processing is executed without using an emergency savingblock; and

FIG. 9 is a diagram illustrating a mounting example of a memory systemaccording to an embodiment.

DETAILED DESCRIPTION

According to the present embodiment, a memory system is provided. Thememory system includes a nonvolatile memory, a control unit, and a powersupply circuit. Write data sent from a host is written into thenonvolatile memory. Furthermore, a volatile memory is disposed in thecontrol unit to temporarily store the write data sent from the host. Inaddition, the control unit controls writing the write data into thenonvolatile memory. Furthermore, the power supply circuit provides witha backup battery to store power sent from an external power supply. Inaddition, the power supply circuit supplies the power sent from theexternal power supply to the nonvolatile memory and the control unit. Ina case of normal operation in which power is normally supplied from theexternal power supply to the memory system, the control unit executes atleast one cache operation and at least one host write operation. In thecache operation, the control unit returns a write completion response tothe host upon completion of reception of the write data from the hostasynchronously with completion of writing the write data into thenonvolatile memory. In the host write operation, the control unit writesthe write data from the host received by the volatile memory into afirst block in the nonvolatile memory in a multiple values. And in acase of unordinary power-off when power sent from the external powersupply suddenly stops, the power supply circuit conducts changeover tooperation using the backup battery. Furthermore, in a case of theunordinary power-off, the control unit writes first dirty data that isincluded in the write data in the volatile memory and that is not yetwritten and that is not completed in writing into the first block into asecond block in the nonvolatile memory with two values. In addition,when next boot after the unordinary power-off has occurred, the controlunit reads the first dirty data from the second block into the volatilememory. Thereafter, the control unit writes the first dirty data in thevolatile memory into a third block in the nonvolatile memory in amultiple values.

Hereafter, a memory system and a controller according to an embodimentwill be described in detail with reference to accompanying drawings. Bythe way, the present invention is not restricted by the embodiment.

Embodiment

FIG. 1 is a diagram illustrating a configuration of a memory systemaccording to an embodiment. In the present embodiment, a case where thememory system is an SSD (Solid State Drive) 1 will be described. In theSSD 1 in the present embodiment, a backup battery 40 is made small byreducing power required to conduct PLP (Power Loss data Protection)processing when sudden power shut down has occurred.

In the SSD 1 in the present embodiment, a dedicated block for emergencysaving (emergency evacuation block 30) is prepared in a NAND 7. In acase where sudden power shut down has occurred, the SSD 1 writes data onvolatile memory (write buffer 26) into the emergency evacuation block 30by using power of the backup battery 40. In addition, the SSD 1 readsdata in the emergency evacuation block 30 onto the volatile memory whennext boot (next activation). And the SSD 1 writes the data on thevolatile memory into a block assigned to host writing in the NAND 7. Theassigned block (hereafter referred to as host write block) is a block towhich a host 2 instructs a controller 5 to write the data.

The SSD 1 is connected to the host (host computer) 2 and an externalpower supply 4. Upon receiving a write command from the host 2, the SSD1 executes write processing of write data (data storage) according tothe write command. The SSD 1 includes the controller 5, a power supplycircuit 6, and a NAND (NAND type flash memory) 7.

The controller 5 includes a semiconductor chip (SoC: System on a Chip)such as, for example, an LSI (Large-Scale Integrated circuit). Thecontroller 5 controls data transfer between the host 2 and the NAND 7,and the like.

The controller 5 includes an FE (Frontend) 10 and a BE (Backend) 20. TheFE 10 sends a write command and write data from the host 2, to the BE20. The BE 20 temporarily stores write data, and writes the temporarilystored write data into the NAND 7.

The FE 10 includes a host I/F 11. Furthermore, the FE 10 includes a CPU(Central Processing Unit) (not illustrated). The CPU controls the FE 10.The host I/F 11 receives data (such as a write command and write data)sent from the host 2. Furthermore, the host I/F 11 transmits data (suchas read data) to the host 2.

The BE 20 includes a LUT (Look up Table) 21, a WC (Write Controller) 22,an ECC (Error Correction Code) 23, a block management unit 24, a CM(Compaction Manager) 25, a write buffer 26, a comp. buffer 27, and aNANDC (NAND Controller) 28. Furthermore, the BE 20 includes a CPU (notillustrated), and the CPU controls the BE 20.

The LUT 21 is an address translation table used to conduct addresstranslation between a logical address specified from the host 2 and aphysical address in the NAND 7. In the LUT 21, a logical address and aphysical address are associated with each other. The LUT 21 is referredto as FTL (Flash Translation Layer) table as well. When reading out datafrom the NAND 7, the ECC 23 corrects an error in data read out.

The write buffer 26 is volatile memory (a data write memory) thattemporarily stores write data instructed to write by the host 2. Thewrite buffer 26 is, for example, SRAM (Static Random Access Memory) andDRAM (Dynamic Random Access Memory).

Data stored in the write buffer 26 is written into the NAND 7 atpredetermined timing. The WC 22 controls buffering processing of writedata into the write buffer 26. If write data is sent from the host 2,the WC 22 causes write data to be stored temporarily in the write buffer26. By the way, in the ensuing description, the write buffer 26 isreferred to as WB 26 in some cases.

The comp. buffer 27 is a buffer in which data of compaction target isstored temporarily when executing compaction (referred to as garbagecollection as well). The compaction is processing of disposing of unusedpages in a block. Owing to execution of the compaction, scattered pagesin use are collected into one physical block and empty blocks aresecured. The CM 25 controls the compaction using the comp. buffer 27.

The block management unit 24 manages respective blocks in the NAND 7.The block management unit 24 sets an emergency evacuation block 30 insome block, for example, in the NAND 7. Furthermore, the blockmanagement unit 24 manages, for example, the number of free blocks,which are blocks into which data can be written. If the number of freeblocks becomes insufficient, the block management unit 24 increases thenumber of free blocks by causing the CM 25 to execute compaction or thelike. The NANDC 28 controls operation of the NAND 7. The NANDC 28executes writing write data into the NAND 7 and reading data from theNAND 7.

The NAND 7 stores write data (user data) specified by the host 2, andstores management data (such as physical block management information)which will be described later. The emergency evacuation block 30 is setin the NAND 7. The emergency evacuation block 30 is a dedicated blockused at the time of PLP processing executed when an unordinarypower-off, which is sudden power shut downped, has occurred. Theemergency saving block may be any block as long as the block can be usedin the NAND 7. The block set to be the emergency evacuation block 30 ischanged according to the use situation of the NAND 7. In the NAND 7, forexample, data is erased by a block unit, and data is written and read bya page unit.

The power supply circuit 6 includes the backup battery 40. The powersupply circuit 6 supplies power sent from the external power supply 4 tothe controller 5 and the NAND 7. Furthermore, the power supply circuit 6supplies power sent from the external power supply 4 to the backupbattery 40. The backup battery 40 is, for example, a capacitor, a supercapacitor, an electrolytic capacitor, a secondary battery, or the like.

The backup battery 40 stores power sent from the external power supply4. When power sent from the external power supply 4 is stopped suddenly(in a case where sudden power shut down has occurred), the backupbattery 40 supplies stored power to the controller 5 and the NAND 7. Thecase where sudden power shut down has occurred is, for example, a casewhere power sent from the external power supply 4 to the power supplycircuit 6 is stopped at timing that is not intended by the user.

At the time of normal operation when power is supplied normally (whensudden power shut down does not occur), the SSD 1 is driven by theexternal power supply 4 in this way. In a case where stopped of theexternal power supply 4 is detected, changeover to drive using thebackup battery 40 is conducted. The “power is supplied normally”, forexample, indicates a state in which the power to the extent that SSD1can perform read/write data to the NAND 7 in a region other than theemergency evacuation block 30 is supplied from the external power supply4 in SSD1.

When transmitting a read command or a write command to the SSD 1, thehost 2 specifies LBA (Logical Block Addressing) functioning as thelogical address. The LBA is a logical address in which a serial numberbeginning with 0 is given to a sector.

Write data sent from the host 2 and received by the SSD 1 is developedon the write buffer 26 with a logical block managed by the host 2 takenas the unit. Then, as for write data in the write buffer 26, a clusterwhich is a conversion granularity of the LUT 21 is taken as a minimumunit. With a page which is an integer times the cluster taken as theunit, write data in the write buffer 26 is written into the NAND 7.

The SSD 1 in the present embodiment has a PLP function which is aprotection function of write data. The PLP function is a function ofensuring persistence of write data for which a write completion responseis given to the host 2 even in a case where sudden power shut down hasoccurred. In other words, the PLP function is a function of storingwrite data to prevent write data which has been written once when viewedfrom the host 2 from reverting even in a case where sudden power shutdown has occurred.

Furthermore, in a stage in which write data is stored in the writebuffer 26, the SSD 1 gives a write completion response to the host 2before non-volatilization is completed, as a cache operation. Such acache operation is high in write processing as compared with a cache offoperation in which a write completion response is given after waitingfor completion of a program to the NAND 7.

The SSD 1 in the present embodiment non-volatilizes dirty data (data notyet written) in the write buffer 26 by utilizing the backup battery 40when sudden power shut down has occurred, in order to satisfyrequirements of the PLP function. The dirty data is data required to benon-volatilized by the PLP function after sudden power shut down hasoccurred. By the way, in the ensuing description, dirty data in thewrite buffer 26 is referred to as WB dirty. The WB dirty is at least aportion of write data in the write buffer 26.

FIG. 2 is a diagram used to explain an operation processing procedure inthe memory system according to the embodiment. An operation processingprocedure in the SSD 1 in a case where sudden power shut down hasoccurred will be described with reference to FIG. 2. If sudden powershut down has occurred (st1), the BE 20 writes WB dirty, which is datanot yet written into the NAND 7, into the emergency evacuation block 30(st2). As a result, non-volatilization of the WB dirty is conducted. Inthe SSD 1, time elapsed until non-volatilization of the WB dirty isconducted since occurrence of the sudden power shut down is PLPprocessing time.

If next boot is executed in the SSD 1 after the sudden power shut downhas occurred (st3), the BE 20 restores the WB dirty from the emergencyevacuation block 30 to the WB 26. Specifically, the BE 20 reads the WBdirty from the emergency evacuation block 30 and writes the WB dirtyinto the write buffer (WB) 26 (st4).

In addition, the BE 20 writes the WB dirty in the write buffer 26 into ahost write block in the NAND 7 (st5). As a result, the PLP function isimplemented in the SSD 1.

In the present embodiment, the SSD 1 prepares the emergency evacuationblock 30, which is a block dedicated to the PLP function, in the NAND 7in this way. And the SSD 1 makes the WB dirty non-volatile by using theemergency evacuation block 30 only at the time of the PLP processing.

By the way, at the time of next boot which is the processing of st3, inaddition, sudden power shut down occurs in some cases. In this case, theprocessing of st4 and st5 is executed when normal activating isconducted at the time of activating further next time or later.

A processing flow at the time when sudden power shut down has occurredwill now be described. FIG. 3 is a flow chart illustrating a PLPprocessing procedure in the memory system according to the embodimentwhen sudden power shut down has occurred. If sudden power shut downoccurs, the power supply circuit 6 detects power shut down from theexternal power supply 4 (step S11). As a result, the power supplycircuit 6 changes over to drive using the backup battery 40 (step S12).

And the power supply circuit 6 notifies the FE 10 and the BE 20 of apower shut down event which indicates that sudden power shut down hasoccurred (step S13). As a result, each of the FE 10 and the BE 20executes PLP processing (step S14).

Here, a processing procedure of the PLP processing conducted by the FE10 will be described. FIG. 4 is a flow chart illustrating a PLPprocessing procedure conducted by the FE in the embodiment. Uponreceiving the power shut down event (step S21), the FE 10 stopsaccepting a new command from the host 2 (step S22).

In addition, the FE 10 cancels a command in process (step S23). Acommand in process in the FE 10 is a command that is included incommands sent from the host 2 and that is not completed in processing inthe BE 20. Therefore, the command in process in the FE 10 is a commandfor which the FE 10 does not give a completion response to the host 2.

For example, there is a case where sudden power shut down occurs after awrite command is sent from the host 2 to the FE 10. If in this casewriting of write data corresponding to the write command is notcompleted for the BE 20, the write command is a command in process andconsequently the write command is canceled.

A processing procedure of PLP processing conducted by the BE 20 will nowbe described. FIG. 5 is a flow chart illustrating a PLP processingprocedure conducted by the BE in the embodiment. Upon receiving thepower shut down event (step S31), the BE 20 cancels a command in process(step S32).

For example, in the BE 20, there is a case where sudden power shut downoccurs after a write command is sent to the WC 22 to the NANDC 28. If inthis case writing of write data corresponding to the write command isnot completed for the NAND 7, the write command is a command in processand consequently the write command is canceled. By the way, in somecases, a portion of write data corresponding to a write command isnon-volatilized and a remaining portion is not non-volatilized becauseof cancel. Even in this case, a completion response of the write commandis not returned to the host 2, and consequently the cancel processing ispermitted.

After the command in process is canceled, the BE 20 performsnon-volatilization of the WB dirty (step S33). Specifically, the BE 20writes the WB dirty in the write buffer 26 into the emergency evacuationblock 30 in the NAND 7.

Cache processing of write data executed in the write buffer 26 will nowbe described. FIG. 6 is a flow chart illustrating a cache processingprocedure of write data. The FE 10 receives a write command sent fromthe host 2 (step S41). Then, the FE 10 starts processing to receivewrite data sent from the host 2, in the WB 26.

At this time, the WC 22 determines whether there is an empty area (freespace) in the WB 26 (step S42). In a case where an empty area is notfound in the WB 26 (step S42, No), the WC 22 continues retrievalprocessing of an empty area. By the way, in a case where there isn't asufficient empty area in the WB 26, the FE 10 temporarily stopsreception of write data sent from the host 2. If an empty area is foundin the WB 26 (step S42, Yes), the WC 22 secures an empty area in the WB26 (step S43).

And the WC 22 transmits write data to the empty area in the WB 26. As aresult, the WB 26 receives the write data (step S44) and stores thewrite data. The WC 22 updates an address translation table (step S45).The address translation table is a table indicating a correspondencerelation between a logical address specified from the host 2 and anaddress in the WB 26.

The FE 10 determines whether all write data corresponding to the writecommand are received by the WB 26 (step S46). Unless all write data arereceived (step S46, No), the SSD 1 continues the reception processing ofthe write data. In other words, the processing of the steps S42 to S45is executed in the SSD 1.

If all write data are received (step S46, Yes), the FE 10 transmits acompletion response to the host 2 (step S47). All write data for whichthe completion response is transmitted to the host 2 are required tohave persistence. Therefore, the SSD 1 non-volatilizes write data forwhich a write completion response is transmitted to the host 2, by usingthe NAND 7.

By the way, NAND elements included in the NAND 7 include two kinds: SLC(Single-level Cell) and MLC (Multiple-level Cell). In the SLC, data ofone bit (two values) is recorded in one recording element. In the MLC,data of at least two bits (multiple values) is recorded in one recordingelement. And data writing into the SLC is faster in writing speed thandata writing into the MLC.

When writing the write data or the like into the NAND 7, either a methodof writing into the MLC via the SLC (SLC cache operation) or a method ofwriting directly into the MLC (MLC direct write operation) is used.

These SLC cache operation and MLC direct write operation are usedproperly to make a WAF (Write Amplification Factor) small. The WAF is awrite magnification of write data from the host 2 to the NAND 7.Specifically, the WAF indicates what times write processing into theNAND 7 has occurred as compared with write data written from the host 2into the write buffer 26. Therefore, it is desirable that the WAF islower.

(SLC Cache Operation)

In a case where write data stored in the write buffer 26 is to bewritten into the NAND 7, the write data is first written into the SLCwith two values (in a binary). In other words, in case of the SLC cacheoperation, host writing is first conducted with respect to an SLC block.Then, as for the write data written into the SLC block, compaction intoan MLC block is conducted. In case of the SLC cache, the write data iswritten into the SLC once and reliability of the written write data ishigh.

In case of such an SLC cache operation, write processing that is high inlocalization contributes to lowering of the WAF of the MLC writing. Thisis because write processing that is high in localization has a highpossibility of being overwritten in the SLC block (SLC cache).

On the other hand, in write processing that is low in localization(whole area random writing or sequential writing), the WAF becomes largeby using the SLC cache operation. As a result, in the case of the SLCcache operation, write processing that is low in localization shortens alifetime of the NAND elements, makes the bandwidth small, and increasespower dissipation as compared with write processing that is high inlocalization.

(MLC Direct Write Operation)

In the case of the MLC direct write operation, write data stored in thewrite buffer 26 is written directly into the MLC in a multiple values.In other words, in the case of the MLC direct write operation, hostwriting is conducted directly into the MLC block. In the case of writeprocessing that is low in localization, the MLC direct write operationlowers the WAF as compared with the SLC cache operation because there isno writing into the SLC. In a work load of an enterprise that is low inlocalization, therefore, the MLC direct write operation is advantageousas compared with the SLC cache operation to reduce the WAF.

In the NAND 7, for example, an MLC block used for MLC and an SLC blockused for SLC are disposed. By the way, at the time of SLC cacheoperation, two values data may be written into the MLC.

In the SSD 1 in the present embodiment, the MLC direct write operationis conducted at the time of normal operation (steady state in which theexternal power supply 4 is supplied normally). Furthermore, in the SSD1, data writing into the NAND 7 is conducted by two values writing atthe time of PLP processing. Specifically, writing into the SLC isconducted at the time of processing of (st2) in FIG. 2. The writing intothe SLC is shorter in program time than writing into the MLC. In a casewhere sudden power shut down has occurred, therefore, data writing intothe NAND 7 can be conducted with a power quantity (energy) less thanthat at the time of normal operation, in the SSD 1.

At the time of next boot after the PLP processing, the SSD 1 restoreswrite data written into the SLC block to the WB 26 as WB dirty, and thenwrites the write data into the MLC block. In other words, in a casewhere sudden power shut down has occurred, the SSD 1 conducts the SLCcache operation by using the emergency evacuation block 30.

As described above, the SSD 1 writes write data and the like into theemergency evacuation block 30, for example, only at the time of PLPprocessing. In this case, the WC 22 in the SSD 1 previously erases datain the emergency evacuation block 30 at the time of normal operation.Furthermore, in the SSD 1, the emergency evacuation block 30 may be usedin common with a data write destination block used at the time of normaloperation. In this case, the WC 22 selects blocks set in the emergencyevacuation block 30 in order among from blocks in the NAND 7. In otherwords, blocks in the NAND 7 are set in the emergency evacuation block 30according to a shift method. The SSD 1 does not write the write datainto a block that is set in the emergency evacuation block 30, with twovalues at the time of normal operation.

Furthermore, at the time of the PLP processing, with respect to writedata that is included in the WB dirty and that is insufficient for apredetermined size (for example, a cluster size which is a minimum unitof address translation), the SSD 1 may write the write data into theemergency evacuation block 30 without conducting padding processing(fill gap read processing) to obtain data corresponding to the clusterunit.

A minimum unit with which the host 2 can conduct write processing withrespect to the SSD 1 is a first unit (sector). And in the SSD 1,management of the physical position or inputting/outputting of data inthe NAND 7 is conducted by using a second unit (cluster or page) whichis larger than the first unit.

In a case where write data sent from the host 2 is not lined up with thesecond unit or is insufficient for the second unit, therefore, in thenormal operation, merging (coupling) of data is conducted to make thewrite data into the NAND 7 sufficient for the second unit.

Specifically, data of a deficiency of write data is read from the NAND 7or the write buffer 26, and appropriation processing is conducted tosatisfy the second unit with a total size of the data which is read andthe write data from the host 2. In other words, in the paddingprocessing, write data that is insufficient for the cluster unit ismerged with data read from the NAND 7 or the like. As a result, writedata that is insufficient for the cluster unit is made uniform with dataof the cluster unit. And data made uniform with the cluster unit iswritten into the NAND 7.

In a case where the padding processing is not executed at the time ofthe PLP processing, it becomes unnecessary to read data from the NAND 7.As a result, it is possible to prevent error correction that is high incorrection intensity (error correction that is long in correctionprocessing time) from being executed at the time of PLP processing. In acase where padding processing is not executed at the time of the PLPprocessing, the SSD 1 executes padding processing when next boot.

Furthermore, at the time of the PLP processing, the SSD 1 may write metadata indicating the state of the write buffer 26 into the emergencyevacuation block 30. In this case, the meta data is non-volatilized attiming similar to that of the WB dirty.

The meta data is data indicating a storage state of the write datastored in the write buffer 26. The meta data includes, for example,information concerning a range of a logical address of data received bythe write buffer 26.

(Setting Example 1 of Meta Data)

The write buffer 26 is managed by the meta data with a third unit (forexample, 4 kB which is a cluster unit) which is an integer times of thefirst unit (sector). The third unit is the same unit as the managementunit of the LUT 21.

In the meta data, a logical address of received data is stored everythird unit and a flag indicating whether there is data is stored everyfirst unit. In other words, for example, a logical address of everycluster (for example, 4 kB interval) and a bit map (for example, 8 bits)indicating a fill gap state of every sector (for example 512 B) arestored in the meta data.

(Setting Example 2 of Meta Data)

Furthermore, the meta data may be created in the write buffer 26 as aset of sector data corresponding to the write command. In this case, inthe meta data, a range of a logical address (a sector (for example, 512B) interval) is defined every command, and the meta data becomes a setof these ranges. And if areas (ranges of logical address) overlapbetween commands, in the meta data these areas are merged (overwrittenon newer data). The meta data that is set in the setting example 2 ofmeta data is higher in degree of abstraction than the meta data that isset in the setting example 1 of meta data.

Furthermore, the SSD 1 may write the management data in the BE 20 intothe emergency evacuation block 30 at the time of the PLP processing. Inthis case, the management data is non-volatilized at timing similar tothat of the WB dirty.

The management data is data used to manage the state of the SSD 1 (forexample, information indicating the operation state and the like). Notonly in a case where sudden power shut down has occurred, but also in acase where normal power shut down has occurred, the management data isnon-volatilized as occasion demands. Management data that is writteninto the emergency evacuation block 30 at the time of the PLP processingis data that is not finished in non-volatilization and that needsnon-volatilization.

Data that is included in the management data and that is desired to benon-volatilized when sudden power shut down has occurred is dirty data.In other words, dirty data in the management data is a portion that isnon-volatilized at the time of the PLP processing after sudden powershut down has occurred.

Here, the management data will be described. The management data isnon-volatilized by using, for example, a snap shot and a log. And themanagement table includes (1) an address translation table, (2) aneffective data quantity counter, (3) a logical block configurationtable, (4) a physical block management information, (5) a logical blockmanagement information, and (6) a logical block list.

Snap Shot and Log

The snap shot is obtained by storing all or a part of the state of theSSD 1 at a certain time point from among management data of volatilememory (DRAM or SRAM) (hereafter referred to as volatile memory X) inthe controller 5 into the NAND 7. The log is obtained by storing onepiece or a plurality of collected pieces of update information(difference) of an entry of management data in the volatile memory Xwhen a change is made on the entry. For example, in a case where thesnap shot is an address translation table, the log is an updatedifference of the address translation table. At the time of anunordinary power-off (power supply shut down), the SSD 1 writes at leasta portion of the log where non-volatilization is not conducted into theemergency evacuation block 30 as dirty data with two values.

Management data lost from within the volatile memory X by power shutdown can be restored by combining a snap shot at a certain time pointand a log at that time point and later. Specifically, the latest snapshot stored before the power shut down is read into the volatile memoryX. And update information included in a log group in a range from thesnap shot that is read to the time of the power shut down is applied tothe pertinent entry in a time series order. As a result, the pertinententry at the time of the power shut down is obtained. In the presentembodiment, a set of logs that are not yet non-volatilized is dirtydata.

(1) Address Translation Table

The address translation table is a table used to translate a logicaladdress instructed by the host 2 to a physical address in the NAND 7where user data of the logical address is recorded. In other words, inthe address translation table, a logical address specified by the host 2and a physical address in the NAND 7 are associated with each other.

(1-1) Address Translation Table that is not Hierarchical

A non-hierarchical address translation table is non-volatilized by thesnap shot and the log. In the non-hierarchical address translationtable, a log that is not yet non-volatilized is dirty data.

(1-2) Hierarchical Address Translation Table

The address translation table is usually read into the volatile memory Xand used. In a case where the capacity of the SSD 1 is large, however,the address translation table also becomes large, and all of the addresstranslation table cannot be read into the volatile memory X in somecases. Therefore, the address translation table is provided with ahierarchical structure, and all of a first level of hierarchy and aportion or all of a second level of hierarchy are read into the volatilememory X.

(1-2-1) Structure

The first level of hierarchy in the hierarchical address translationtable indicates a correspondence relation between an upper portion ofthe logical address and a physical address in the NAND 7 where thesecond level of hierarchy in the hierarchical address translation tableis recorded. In a case where the first level of hierarchy in thehierarchical address translation table is referred to, therefore, if anupper portion of the logical address is input, a physical address in theNAND 7 where the second level of hierarchy in the hierarchical addresstranslation table is recorded is output.

The second level of hierarchy in the hierarchical address translationtable indicates a correspondence relation between a lower portion of thelogical address and a physical address in the NAND 7 where user data isrecorded. In a case where the second level of hierarchy in thehierarchical address translation table is referred to, therefore, if alower portion of the logical address is input, a physical address in theNAND 7 where the user data is recorded is output. Owing to such aconfiguration of the hierarchical address translation table, translationfrom a logical address to a physical address is conducted as a whole.

(1-2-2) Reference Method

Reference to the hierarchical address translation table is conductedwith the following points. In a case where the second level of hierarchyin the hierarchical address translation table including an entry ofreference target is not developed in the volatile memory X, the firstlevel of hierarchy in the hierarchical address translation table isreferred to (a). And a physical address on the NAND 7 of the secondlevel of hierarchy in the hierarchical address translation table isacquired (b). The second level of hierarchy in the hierarchical addresstranslation table is read from the physical address into the volatilememory X (c).

By the way, in a case where the second level of hierarchy is alreadydeveloped into the volatile memory X, the above-described processing (a)to (c) is skipped. After the second level of hierarchy is read into thevolatile memory X, in the SSD 1, an entry of reference target isreferred to, and a desired physical address is acquired on the basis ofthe entry.

(1-2-3) Update and Non-Volatilization Method

Update and non-volatilization of the hierarchical address translationtable are conducted with the following points. In a case where it isdesired to update an entry including the hierarchical addresstranslation table, if the second level of hierarchy in the addresstranslation table including the entry of update target is not developedinto the volatile memory X, development is conducted. This processing isprocessing similar to that in the above-described (a) to (c).

In the second level of hierarchy developed in the volatile memory X, anentry of update target is updated. Thereafter, the second level ofhierarchy in which update has occurred is recorded is recorded in theNAND 7. By the way, this recording may not be conducted immediatelyafter the update. After recording of the second level of hierarchy, aphysical address in the NAND 7 is written in the pertinent entry in thefirst level of hierarchy on the volatile memory X (B).

By the way, the first level of hierarchy is non-volatilized by using thesnap shot and the log. As the snap shot, the whole of the first level ofhierarchy on the volatile memory X at a certain time point is writteninto the NAND 7. As the log, update (B) of one entry unit of the firstlevel of hierarchy is written into the NAND 7.

(1-2-3) Dirty Data

Dirty data in the hierarchical address translation table is a set of thesecond level of hierarchy in which update has occurred butnon-volatilization is not completed and a set of logs in the first levelof hierarchy.

(2) Effective Data Quantity Counter (Effective Cluster Counter)

In order to conduct compaction efficiently, an effective data quantity(a quantity of data that is not overwritten) included in the pertinentlogical block is counted every logical block described later. This countvalue is the effective data quantity counter. Since the counter group iscomparatively small in data size, the counter group is stored, forexample, by the snap shot at the time of PLP processing. Therefore, theeffective data quantity counter corresponding to all logical blocks inthe SSD 1 becomes dirty data. In other words, the whole of the effectivedata quantity counter becomes dirty data. By the way, the effective dataquantity counter is especially high in update frequency, andconsequently the effective data quantity counter is managed separatelyfrom logical block management information described later.

(3) Logical Block Configuration Table (L2P Table: Logical to PhysicalTable)

In order to improve performance of the SSD 1 and secure reliability ofthe SSD 1, a plurality of physical block groups capable of operating inparallel are collected as a set and managed as a logical block. Thelogical block configuration table indicates an address group of physicalblocks included in each logical block. In other words, the logical blockconfiguration table is information defining a set of a plurality oflogical blocks.

The configuration of a logical block is rearranged due to, for example,occurrence of a faulty physical block in some cases. Therefore, thelogical block configuration table is non-volatilized by the snap shotand log. Dirty data in the logical block configuration table is a set oflogs in which update occurs but non-volatilization is not yet completed.

(4) Physical Block Management Information

The physical block management information is information indicatingstates of physical blocks and is defined every physical block. Thephysical block management information is non-volatilized by the snapshot and log. Dirty data in the physical block management information isa set of logs in which update occurs but non-volatilization is not yetcompleted.

(4-1) Faulty State

In the physical block management information, a faulty state indicatingwhether each physical block is a faulty block is managed. By the way, inthe physical block management information, a faulty page group includedin each physical block may be managed.

(4-2) Write Classification

Furthermore, the physical block management information includes writeclassification such as SLC/MLC/TLC (Triple-level Cell).

(4-3) Randomizer Key

When data is written into the NAND 7, randomizing (scrambling) isapplied with the object of dissolving reliability lowering caused byoffset of a bit pattern. A seed used in encode processing and decodeprocessing at this time is a randomizer key, and a portion thereof ismanaged by the physical block management information.

(4-4) ECC Setting (Dynamic ECC)

In the SSD 1, an ECC having a different intensity is used depending uponreliability of every physical block. In the physical block managementinformation, information concerning setting of an error correctionintensity of every set of physical blocks is included. Specifically,setting of frame length and code rate of the ECC is included in thephysical block management information.

(4-5) Read Voltage Threshold Setting

In the SSD 1, phenomena such as program disturb/read disturb/dataretention occur depending upon exhaustion of the physical block andelapsed time after programming. The threshold distribution of the celldeviates from a regular threshold due to the phenomena in some cases.Therefore, a threshold to be used to read each physical block is set asa read voltage threshold. The read voltage threshold of each physicalblock is included in the physical block management information. By theway, information used to correct a deviation of the threshold may be theread voltage threshold itself or a deviation quantity from the originalvoltage.

(4-6) The Number of Times of Erasing

The number of times of erasing is the number of times each physicalblock was erased. The number of times of erasing of each block isincluded in the physical block management information.

(4-7) The Number of Times of Reading Counter

The number of times of reading counter is the number of times of readingconducted on the pertinent physical block since the physical block wasprogrammed. The number of times of reading counter is used to estimatethe state of advance of the read disturb. The number of times of readingcounter is included in the physical block management information.

(4-8) The Number of Times of Error

The number of times of error is the number of times of erase error, thenumber of times of program error, and the like which have occurred ineach physical block. The number of times of error is used as, forexample, a measure of faulty block occurrence. The number of times oferror is included in the physical block management information.

By the way, it is not necessary that all of the above-described faultystate, write classification, randomizer key, ECC setting, read voltagethreshold setting, the number of times of erasing, the number of timesof reading counter, and the number of times of error are included in thephysical block management information. At least one of information kindsof (4-1) to (4-8) described above is included in the physical blockmanagement information.

(5) Logical Block Management Information

The logical block management information is information indicating astate and the like of a logical block, and is defined every logicalblock. The logical block management information is non-volatilized bythe snap shot and log. Dirty data in the logical block managementinformation is a set of logs in which update occurs butnon-volatilization is not yet completed.

(5-1) Logical Block State

In the logical block state, it is indicated that each logical block isin which of states of already erased/in writing/after writecompletion/free (after compaction). The logical block state is includedin the logical block management information.

(5-2) Logical Block Use

The logical block use indicates the use of the logical block is which ofuser data storage, address translation table storage, and other systemdata storage. The logical block use is included in the logical blockmanagement information.

(5-3) Write Completion Page Number

The write completion page number is a number of a page where writing iscompleted last in a logical block. The write completion page number isincluded in the logical block management information.

By the way, it is not necessary that all of the above-described logicalblock state, logical block use, and write completion page number areincluded in the logical block management information. At least one ofthe information kinds (5-1) to (5-3) described above is included in thelogical block management information.

(7) Logical Block List

The logical block list is a list used to manage a set of logical blocks.The logical block list includes a list of logical blocks of user dataafter write completion, a list of logical blocks in the addresstranslation table after write completion, and a list of free blocks.

Since the logical block list is comparatively small, the logical blocklist is stored by using the snap shot, for example, at the time of thePLP processing. Therefore, the whole of the logical block list becomesdirty data.

By the way, it is not necessary that all of the above-described (1)address translation table, (2) effective data quantity counter, (3)logical block configuration table, (4) physical block managementinformation, (5) logical block management information, and (6) logicalblock list are included in the management data. At least one of theabove-described information kinds (1) to (6) is included in themanagement data.

Flush processing of the WB dirty at the time of the normal operationwill now be described. FIG. 7 is a flow chart illustrating a processingprocedure of flush processing at the time of the normal operation. InFIG. 7, the flush processing procedure of WB dirty at the time of normaloperation is illustrated.

Write data from the host 2 is written into the write buffer 26, and thenwritten into the NAND 7. In this case, in the write buffer 26, an areain the write buffer 26 where data deletion is made possible by writingthe write data into the NAND 7 is released. This release processing isflush processing.

At the time of the normal operation, the WC 22 in the BE 20 startspadding processing on a portion in the WB dirty that is insufficient forthe cluster unit (step S51). And the WC 22 in the BE 20 refers to theaddress translation table and acquires a physical address from thelogical address of the WB dirty (step S52). And the WC 22 in the BE 20reads data of a deficiency size from the NAND 7 (step S53). Data of thedeficiency size is data to be used to compensate for a size of a portionthat is insufficient for the cluster unit. The size is determineddepending upon the size of the portion that is insufficient for thecluster unit.

The ECC 23 in the BE 20 conducts error correction processing on the dataof the deficiency size read from the NAND 7 (step S54). In addition, theWC 22 in the BE 20 merges the WB dirty that is insufficient for thecluster unit and data of the deficiency size read from the NAND 7 (stepS55). In this way, the WB dirty that is insufficient for the clusterunit and data of the deficiency size are merged and united. As a result,data of cluster unit is generated.

Then, the WC 22 in the BE 20 writes the WB dirty after the merge intothe host write block (step S56). At this time, the WB dirty is writteninto the NAND 7 in a multiple values. And in the BE 20, the addresstranslation table is updated (step S57). In addition, in the BE 20,release of the write buffer 26 is conducted (step S58).

(In the case of Normal Operation)

As described heretofore, the SSD 1 in the present embodiment executesthe following processing as at least one cache operation and at leastone host write operation at the time of the normal operation. In otherwords, in a case where the cache operation is conducted, the SSD 1receives write data by using the write buffer 26 in response to a writecommand from the host 2. And upon completion of reception of write datacorresponding to the write command, the SSD 1 returns a write completionresponse to the host 2 asynchronously with completion of writing thewrite data into the NAND 7. Furthermore, in a case where host writeoperation is conducted, the SSD 1 writes write data received from thehost 2 by using the write buffer 26, into a block (host write block) onthe NAND 7 assigned to host writing in a multiple values.

(In the Case of PLP Processing)

Furthermore, upon detecting stop of the external power supply 4 at thetime of the PLP processing, the SSD 1 in the present embodiment conductschangeover to operation using the backup battery 40. And the SSD 1writes data (WB dirty) that is included in data received from the host 2by the write buffer 26 and that is not completed in writing into thehost write block into the emergency evacuation block 30 with two values.

(In the Case of Activation)

Furthermore, at the time of the next boot after the PLP processing, theSSD 1 in the present embodiment reads the WB dirty in the emergencyevacuation block 30 into the write buffer 26. And the SSD 1 writes theWB dirty in the write buffer 26 into the host write block on the NAND 7in a multiple values by using power supplied from the external powersupply 4. At this time, the SSD 1 may write the WB dirty in the writebuffer 26 into a host write block different from the host write blockused before the stop of the external power supply 4 is detected, in amultiple values. Owing to these kinds of processing, it becomes possibleto store the WB dirty into the NAND 7 with low power even in a casewhere sudden power shut down has occurred.

Here, PLP processing that does not use the emergency evacuation block 30will be described. FIG. 8 is a flow chart illustrating a PLP processingprocedure in a case where PLP processing is executed without using theemergency saving block. With reference to FIG. 8, an operationprocessing procedure of the SSD 1 in a case where sudden power shut downhas occurred will be described.

If sudden power shut down has occurred (st11), the BE 20 starts paddingprocessing on a portion of the WB dirty that is insufficient for thecluster unit (st12). And the WC 22 in the BE 20 acquires a physicaladdress from a logical address of the WB dirty in the addresstranslation table (st13). In addition, the WC 22 in the BE 20 reads dataof a deficiency size from within the NAND 7 (st14).

The ECC 23 in the BE 20 conducts error correction processing on the dataof the deficiency size read from the NAND 7 (st15). In addition, the WC22 in the BE 20 merges WB dirty which is insufficient for the clusterunit and data of the deficiency size read from the NAND 7 (st16). Inthis way, the WB dirty that is insufficient for the cluster unit and thedata of the deficiency size are merged and united. As a result, data ofthe cluster unit is generated.

Then, in the BE 20, the WB dirty subjected to padding processing iswritten into the host write block in a multiple values (st17). As aresult, non-volatilization of the WB dirty is conducted.

If padding processing is conducted on write data that is insufficientfor the cluster unit and then the PLP processing is executed in thisway, error correction processing is conducted on data of the deficiencysize read from the NAND 7. If in this case error correction that is highin error correction intensity is executed, it takes a long time toconduct the PLP processing. The error correction that is high in errorcorrection intensity is, for example, repeated correction processing(such as L3 correction) of a product code using an intra-page parity andan inter-page parity. In a case where error correction that is high inerror correction intensity is executed, it becomes difficult to ensure amaximum time required to conduct the PLP processing.

Furthermore, when writing the WB dirty subjected to the paddingprocessing into the host write block, if the WB dirty is written intothe MLC in a multiple values, it takes a long time to program into theMLC. Furthermore, in PLP processing in the case where the emergencyevacuation block 30 is not used, time from occurrence of sudden powershut down to non-volatilization of the WB dirty is PLP processing time.Therefore, the PLP processing in the case where the emergency evacuationblock 30 is not used takes a longer time as compared with the PLPprocessing in the case where the emergency evacuation block 30 is used.

A mounting example of the SSD 1 which is a memory system will now bedescribed. FIG. 9 is a diagram illustrating a mounting example of amemory system according to an embodiment. The SSD 1 is mounted on aserver system 100. The server system 100 includes a disk array 200 and arack mount server 300 connected by a communication interface 400. Asstandards of the communication interface 400, arbitrary standards can beadopted. The rack mount server 300 includes at least one host 2 mountedon a server rack.

Furthermore, the disk array 200 includes at least one SSD 1 and at leastone hard disk unit 8 mounted on a server rack. The disk array 200includes a power supply which is not illustrated. Power from the powersupply is supplied to units mounted on the disk array 200 via a backplane which is not illustrated.

By the way, in the disk array 200, for example, at least one memorysystem 1 is used as a cache of at least one hard disk unit. In the diskarray 200, a storage controller unit constructing a RAID may be mountedon at least one hard disk unit 8.

By the way, the SSD 1 may conduct SLC cache operation at the time of thenormal operation. Furthermore, the SSD 1 may execute MLC direct writeoperation when sudden power shut down has occurred. Furthermore, whensudden power shut down has occurred, the SSD 1 may write the write datainto the NAND 7 with two values without using the emergency evacuationblock 30. In this case, the SSD 1 may write the write data and themanagement data collectively into the NAND 7, or may write the writedata and the meta data collectively into the NAND 7.

Furthermore, in a case where sudden power shut down has occurred, theSSD 1 is not restricted to writing the WB dirty into the emergencyevacuation block 30 with two values, but the SSD 1 may conduct writingby using any method as long as writing into the emergency evacuationblock 30 can be conducted with lower power as compared with the normaloperation.

Furthermore, the SSD 1 may conduct data writing into the host writeblock with two values until the charging quantity of the backup battery40 reaches a predetermined quantity since power supply from the externalpower supply 4 to the SSD 1 is turned on.

Furthermore, when sudden power shut down has occurred, the SSD 1 maynon-volatilize the write data by conducting two values data writing intoa block that is subject to multiple values writing at the time of normaloperation. In this case, a write page written with two values isadditionally recorded (post-scripted) to the block that is subject tomultiple values writing at the time of normal operation.

Furthermore, data written into the emergency evacuation block 30 at thetime of PLP processing is not restricted to WB dirty, the managementdata, and the meta data, but may be other saving data. Furthermore, thememory system is not restricted to the SSD 1, but may be another devicesuch as a HDD (Hard Disk Drive).

In the embodiment, write data is written into the NAND 7 in a multiplevalues at the time of the normal operation in this way. Furthermore,when sudden power shut down has occurred, the WB dirty is written intothe emergency evacuation block 30 with two values. In addition, at thetime of next boot, the WB dirty is read from the emergency evacuationblock 30 into the write buffer 26, and the WB dirty in the write buffer26 is written into the NAND 7 in a multiple values.

When sudden power shut down has occurred, therefore, the WB dirty iswritten into the emergency evacuation block 30 with lower power ascompared with the time of the normal operation. As a result, the SSD 1can non-volatilize the WB dirty with low power while ensuring the PLPrequirements. Therefore, the battery quantity stored in the backupbattery 40 can be reduced. Consequently, the backup battery 40 can bemade small. Accordingly, the cost of the backup battery 40 can be helddown and the size of the SSD 1 can be made small.

Furthermore, since the WB dirty is non-volatilized by two values writingthat is faster as compared with the time of the normal operation, the WBdirty can be non-volatilized in a short time. Therefore, the backupbattery 40 can be made further small.

Furthermore, writing into the emergency evacuation block 30 is conductedwithout conducting padding processing on write data that is included inthe WB dirty and that is insufficient for the cluster unit. Accordingly,the WB dirty can be non-volatilized in a short time. Therefore, thebackup battery can be made further small.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A memory system comprising: a nonvolatile memory;a buffer; a battery configured to store power on the basis of powersupplied from an external supply, and a controller configured to: in acase where power becomes unsupplied from the external supply, by usingthe power stored in the battery, stop accepting new write commands froma host, cancel a first write command received from the host if acompletion response for the first write command has not been returned tothe host, and write second data stored in the buffer into thenonvolatile memory if a completion response for a second write commandreceived from the host has been returned to the host, the second databeing data related to the second write command.
 2. The memory systemaccording to claim 1, wherein, by cancelling the first write command, atleast a part of first data is not written into the nonvolatile memory,the first data being data related to the first write command.
 3. Thememory system according to claim 1 further comprising a power controlcircuit that includes the battery, wherein the power control circuit isconfigured to supply, to a component of the memory system, at least oneof the power supplied from the external supply and the power stored inthe battery.
 4. The memory system according to claim 1, wherein thenonvolatile memory includes a memory cell, and the controller isconfigured to write the second data into the nonvolatile memory so thatthe memory cell into which at least a part of the second data is writtenstores only one bit.
 5. The memory system according to claim 4, whereinthe nonvolatile memory further includes a plurality of blocks, each ofthe plurality of blocks being a unit for a data erase operation, a firstblock among the plurality of blocks including the memory cell, and thecontroller is further configured to: before the power becomes unsuppliedfrom the external supply, by using the power supplied from the externalsupply, perform a data erase operation to the first block; and in thecase where the power becomes unsupplied from the external supply, byusing the power stored in the battery, write the at least a part of thesecond data into the memory cell in the first block.
 6. The memorysystem according to claim 1, wherein the controller is furtherconfigured to, upon a start-up of the memory system after the controllerhas written the second data into the nonvolatile memory, read the seconddata from the nonvolatile memory using the power supplied from theexternal supply, and write third data into the nonvolatile memory usingthe power supplied from the external supply, the third data being thesecond data read from the nonvolatile memory.
 7. The memory systemaccording to claim 6, wherein the nonvolatile memory includes aplurality of memory cells including a first memory cell and a secondmemory cell, and the controller is configured to: write the second datainto the nonvolatile memory so that the first memory cell into which atleast a part of the second data is written stores only one bit, andwrite the third data into the nonvolatile memory so that the secondmemory cell into which at least a part of the third data is writtenstores more than one bit.
 8. The memory system according to claim 1,wherein the controller is further configured to: manage an addresstranslation table to conduct address translation from a logical addressof data specified by the host to a physical address specifying alocation of the nonvolatile memory in which the data is stored, store,at a first timing, a snapshot of the address translation table into thenonvolatile memory, store, at a second timing after the first timing, alog into the buffer, the log indicating a difference of the addresstranslation table from the snapshot, and in the case where the powerbecomes unsupplied from the external supply, by using the power storedin the battery, write the log stored in the buffer into the nonvolatilememory.
 9. The memory system according to claim 8, wherein, upon astart-up of the memory system after the controller has written the loginto the nonvolatile memory, the controller reads the snapshot and thelog from the nonvolatile memory, and restores the address translationtable of the second timing by using the read snapshot and the read log.10. The memory system according to claim 1, wherein the battery iscomposed of a capacitor.
 11. A solid state drive (SSD) comprising: aNAND flash memory; a capacitor configured to store energy on the basisof energy supplied from a power source; and an SSD controller configuredto: if a completion response for a first write command received from ahost has not been returned to the host in a case where energy becomesunsupplied from the power source, by using energy discharged from thecapacitor, stop all new operations to the NAND flash memory for thefirst write command and abort the first write command received from thehost.
 12. The solid state drive according to claim 11, wherein, the SSDcontroller is further configured to, in the case where the energybecomes unsupplied from the power source, stop accepting new writecommands from the host.
 13. The solid state drive according to claim 11,wherein, by stopping all new operations to the NAND flash memory for thefirst write command, at least a part of first data is not committed tothe NAND flash memory, the first data being data related to the firstwrite command.
 14. The solid state drive according to claim 11 furthercomprising a circuit configured to supply, to a component of the solidstate drive, at least one of the energy supplied from the power sourceand the energy stored in the capacitor.
 15. The solid state driveaccording to claim 11, further comprising: a volatile memory, whereinthe SSD controller is further configured to, in the case where theenergy becomes unsupplied from the power source, commit data-in-flightbuffered in the volatile memory to the NAND flash memory using theenergy discharged from the capacitor, the data-in-flight being dataacknowledged as written back to the host but not yet committed to theNAND flash memory.
 16. The solid state drive according to claim 15,wherein the SSD controller is configured to commit the data-in-flight tothe NAND flash memory in a single level cell (SLC) mode.
 17. The solidstate drive according to claim 15, wherein the NAND flash memoryincludes a plurality of blocks, each of the plurality of blocks being aunit for a data erase operation, and the SSD controller is furtherconfigured to: before the energy becomes unsupplied from the powersource, by using the energy supplied from the power source, perform adata erase operation to a first block among the plurality of blocks; inthe case where the energy becomes unsupplied from the power source, byusing the energy discharged from the capacitor, commit thedata-in-flight to the first block.
 18. The solid state drive accordingto claim 15, wherein the SSD controller is further configured to, upon astart-up of the solid state drive after the SSD controller has committedthe data-in-flight to the NAND flash memory, read the data-in-flightfrom the NAND flash memory, and commit the read data-in-flight to theNAND flash memory in a multi level cell (MLC) mode.
 19. The solid statedrive according to claim 11, further comprising, a dynamic random accessmemory (DRAM), wherein the SSD controller is further configured to:manage a logical-to-physical address table in real-time using the DRAM;and in the case where the energy becomes unsupplied from the powersource, by using the energy discharged from the capacitor, commit, tothe NAND flash memory, information for rebuilding thelogical-to-physical address table.
 20. The solid state drive accordingto claim 19, wherein, the SSD controller is further configured to, upona start-up of the solid state drive after the SSD controller hascommitted the information to the NAND flash memory, rebuild thelogical-to-physical address table from the information.